Products known as pressure washers are used for a wide variety of washing applications. A few such applications include washing building walls, removing paint and stains, washing such items as bicycles and golf carts, and cleaning sidewalks and driveways. Some pressure washers incorporate a venturi device whereby chemical cleaning liquids may be aspirated into and mixed with the water stream. Pressure washers are used by contractors, homeowners and equipment rental businesses, as examples. Generac Corporation, Waukesha, Wis., U.S.A. is a leading manufacturer of pressure washers.
The primary components of a pressure washer include a water pump connected to a source of water such as a garden hose and providing a high-pressure stream or spray of water. The pump is coupled to and driven by a prime mover such as an electric motor or an internal combustion engine.
Because of its high pressure capability, a pump of the reciprocating piston type is usually chosen for pressure washer service. An exemplary piston pump has several (e.g., three) pistons reciprocally moving in respective bores of the pump barrel. The pistons are reciprocated by a rotating "wobble plate"-type cam powered by the prime mover.
A well known water pump arrangement has a housing, the wobble plate cavity of which contains a quantity of oil for lubricating bearings and for maintaining the proximal portions of the reciprocating pistons at an acceptably-low operating temperature, e.g., 260.degree.-270.degree. F.
The cavity and the distal portions of the pistons (which come in contact with and deliver high-pressure water out of the pump) are isolated from one another by appropriate seal arrangements. Such arrangements form a barrier between the oil in the cavity and around the proximal ends of the pistons and the water being pumped by the distal ends of the pistons.
A high pressure water pump represents a severe operating environment for the pistons in it. Special care must be taken to design pistons which not only have acceptable life in the application but which also evidence a cost consistent with the cost and selling price of the pressure washer equipment.
One type of known piston is made of stainless steel with a thin plasma-applied ceramic coating or is made of hardened steel with a phosphatized or "Parkerized" surface treatment. It is believed that ceramic coating of pistons was implemented because one or more of the seals being used therewith is inordinately abrasive and quickly attacks the piston surface finish during pump operation.
Ceramic pistons are disclosed in a number of patent documents including U.S. Pat. No. 4,759,110 (Rieger et al.). The Rieger et al. patent discloses a process for attaching a metal "holder" (shoe) to a ceramic piston. At ambient temperature, the bore of the shoe has a diameter smaller than that of the piston. Attachment is by heating the holder until its bore diameter is greater than that of the piston, placing the pieces together and allowing the shoe to cool.
The patent explains that "optimum" locking of the piston and the holder to one another occurs when (a) the bore and piston diameters are each within a particular range, and (b) the piston end and the holder bore are each prepared to have a particular roughness Ra.
As to structural shape, the end portion of the holder which is perpendicular to the piston long axis is flat. The patent notes that such holder has a relief notch" in order that "uniform stresses are produced in the region of clamping."
U.S. Pat. No. 5,038,673 (Schulze), shows a pump piston, the body of which is ceramic. In the version shown in FIGS. 1, 3a and 4, the piston body has a groove at the head end and a cap-shaped metal contact element over such end. In the arrangements of FIGS. 6, 7, 8, 10, 11 and 14, the contact element is a metal button on the piston end.
U.S. Pat. No. 5,094,150 (Russner et al.) discloses a ceramic piston to which is attached what the patent calls a "metallic drive part," i.e., a shoe. In one arrangement, the ceramic is metallized and the shoe is solder-attached using relatively-common solder. In another arrangement, there is no metallization and the shoe is solder-attached using solder containing, e.g., titanium or zirconium.
U.S. Pat. No. 5,392,693 (Engel et al.) describes a piston assembly having a ceramic body with a spherical head over which is fitted a slipper or shoe, also made of ceramic. As shown in FIGS. 4-6, the piston may be made in two parts, a body and a head, to permit a ceramic ring to be slipped over the shaft of the inverted-key-hole-shaped head.
While known pistons for pressure washer pumps have been generally satisfactory for the purpose, at least those having plural-layer coated construction are not without disadvantages. Chief among them is cost. Such pistons require special, higher-cost materials (stainless or hardened steel) and a special, separately-provided surface coating or treatment. Such coatings and treatments have adverse implications for the manufacturing process and for the manufactured cost of the ready-to-assemble piston.
And pistons made of steel have mass (and, therefore, inertia) which, in view of the invention, is relatively high. Depending in part upon the rotational speed at which the pump is being driven, high-mass pistons may have adverse implications for the size and type of piston return spring and/or for the bearings used in conjunction with the wobble-plate type cam.
An improved piston and method for making such piston and a water pump having an improved piston, all of which address some of the disadvantages and shortcomings of the prior art would represent distinct technological advances in the art.